In an Internet Protocol version 6 (Internet Protocol version 6, IPv6) network, to improve forwarding efficiency of a network device, an intermediate device such as a router no longer has a fragmentation function, and a data packet whose length exceeds a length of a maximum transmission unit (Maximum Transmission, MTU) on an interface of this device is directly discarded. Therefore, a fragment reassembly function of the data packet can only be completed on a host (HOST) node at a transmit end. In other words, only the host node at the transmit end can perform fragmentation on the data packet. Therefore, the host node at the transmit end needs to probe a path maximum transmission unit (Path Max Transmission Unit, PMTU) of a path, to determine a length of a sent data packet.
A current data packet sending process is shown in FIG. 1. The host node at the transmit end uses an egress interface to send a data packet, and an MTU preconfigured on the egress interface is 1500 bytes. When sending a data packet, the host node at the transmit end sends the data packet according to the preconfigured MTU that is 1500 bytes.
After receiving the data packet sent by the transmit end, a router A determines that an MTU configured on an interface for sending the data packet by the router A is 1400 bytes. Therefore, after receiving the data packet whose MTU is 1500 bytes, the router A discards the data packet, sends a message of an Internet Control Message Protocol (internet control message protocol, ICMP) version (version, v) 6 (type=2) to the transmit end to indicate that the received data packet is excessively long, and adds information indicating that the MTU is 1400 bytes to the message.
After receiving the message, the transmit end subsequently sends a data packet whose MTU is 1400 bytes on the path according to the information indicating that the MTU is 1400 bytes in the message.
The subsequent data packet arrives at a router B after passing through the router A. After receiving the data packet whose MTU is 1400 bytes, the router B determines that an MTU configured on an interface used to send the data packet by the router B is 1300 bytes, and therefore, the router B discards the received data packet whose MTU is 1400 bytes, returns the message of ICMPv6 (type=2) to the transmit end to indicate that the received data packet is excessively long, and adds information that indicates that the MTU is 1300 bytes to the message.
After receiving the message, the transmit end subsequently sends a data packet whose MTU is 1300 bytes according to the information indicating that the MTU is 1300 bytes in the message. The data packet whose MTU is 1300 bytes may arrive at a destination node after separately passing through the router A and the router B.
It can be learned from the process shown in FIG. 1 that, the data packet whose MTU is 1500 bytes and the data packet whose MTU is 1400 bytes that are sent by the transmit end are respectively discarded by the router A and the router B. That is, before the transmit end probes a PMTU that is 1300 bytes of a path, all sent data packets are discarded.
In conclusion, in the current IPv6 network, it is possible that because a length of a sent data packet exceeds an MTU length supported by an interface of a network device on a path, the data packet is always discarded before a PMTU of the path is probed by a data packet transmit end.